This invention relates to fuel cell systems and, in particular, to fuel cell systems in which fuel gas for the system is generated by reforming the hydrocarbon content of fuel from a fuel supply.
In fuel cell systems, fuel cells are utilized as a means of generating electricity. In a fuel cell, a fuel process gas such as hydrogen is galvanically combusted with an oxidant process gas, usually oxygen, to form electricity and water. In most fuel cell systems of commercial interest, the fuel process gas or hydrogen is derived from a fossil fuel (i.e., one having hydrocarbon content) such as natural gas, oil or even coal by a chemical reaction. The most common chemical reaction is steam reforming wherein steam and the fuel are reacted in a separate fuel reformer to form hydrogen and carbon dioxide. This latter reaction is endothermic.
Since it is impractical for most fuel cells operating on a fuel process gas containing a mixture of hydrogen and carbon dioxide to consume all of the hydrogen, the usual practice is to recycle the unused hydrogen-carbon dioxide mixture to a burner where heat is recovered. This heat is then used to heat the fuel reformer to thereby sustain the endothermic reforming reaction. In present day systems, the highest utilization or consumption of hydrogen process gas in a fuel cell is about 85%, so that the balance of the hydrogen process gas is used to generate heat for the fuel reformer.
Not all fuel cells, however, require an external fuel processor or reformer. U.S. Pat. No. 3,488,266 describes a fuel cell system in which the fuel processing may be effected directly in the anode chamber of the fuel cell. In the disclosed system, the unused process gas contained in the anode exhaust or tail gas is utilized in a manner similar to that of the external reforming systems. Thus, after the gas is passed through a preheater for the incoming fuel, it is then passed to an external unit which recovers waste heat for steam generation. Alternatively the unused gas may be burned in order to generate waste heat for the preheater.
U.S. Pat. No. 4,182,795 discloses another fuel cell system in which internal reforming is carried out in electrolyte-isolated channels in a stack of high temperature fuel cells such as, for example, molten carbonate fuel cells. In this case, waste heat from the fuel cell is used to react fuels such as methanol and methane with water to form hydrogen process gas internally of the cells. This is made possible because the molten carbonate fuel cells operate at sufficiently high temperatures to support the endothermic reforming heat quality and because the quantity of heat produced by the fuel cell stack is in excess of that required by the reforming reaction.
In the system of the '795 patent, however, it is still impractical to consume more than about 85 or 90% of the hydrogen process gas formed. This difficulty arises because the partial pressure of the hydrogen drops to a low level. This can be exacerbated by slight maldistributions between cells when a large number of cells are common manifolded into a stack. The anode exhaust gas of one cell thus contains substantial amounts of unused hydrogen while other cells are starved for fuel.
In the '795 system, instead of using the exhaust gas for waste heat recovery, as in the '226 patent, it is recirculated back to the anode. It has been found, however, that the recirculated exhaust gas is dilute and leads to lower cell performance, since the recycling tends to build up the concentration of carbon dioxide and water if the latter is not removed by condensation.
It is also necessary with molten carbonate fuel cells to recycle the carbon dioxide produced at the anodes of the cells to the cathodes of the cells where it is needed as a reactant in accordance with the reaction: EQU 2e+CO.sub.2 +1/2O.sub.2.sup.= .fwdarw.CO.sub.3.sup.= (a)
This recycle is usually accomplished by completing the combustion of a portion of the anode exhaust gas so that it contains no fuel process gas, i.e., no hydrogen or carbon monoxide, and mixing the combusted anode exhaust gas with the fresh air required to supply the cathode oxygen in accordance with reaction (a). Unfortunately, by this means a very dilute cathode reactant gas is formed. In particular, the concentrations of carbon dioxide and oxygen needed for reaction (a) are very low because of all the dilution caused by the nitrogen in the air. The low partial pressure of reactants lowers the system performance.
It is therefore an object of the present invention to provide a fuel cell system having enhanced characteristics
It is a further object of the present invention to provide a fuel cell system wherein fuel process gas is more efficiently used.
It is a further object of the present invention to provide a fuel cell system wherein internal reforming is used to produce hydrogen fuel process gas and wherein utilization of the hydrogen gas is increased.
It is yet a further object of the present invention to provide a fuel cell system of the latter type wherein the partial pressure of the hydrogen process gas is increased and wherein the partial pressure of the oxidant process gas and carbon dioxide entering the fuel cell anode is also increased.